1. Field of the Invention
The present invention relates to a latent electrostatic image developing carrier, a process cartridge and an image forming apparatus.
2. Description of the Related Art
In recent years, technologies of copiers and printers based on electrophotography have rapidly been changed from monochromatic modes to full-color modes, and the market of full-color modes tends to expand. In color image formation by full-color electrophotography, a latent electrostatic image is formed on a latent electrostatic image bearing member. Next, the latent electrostatic image is developed with charged three color toners of yellow, magenta and cyan or with charged four color toners of yellow, magenta, cyan and black to thereby form toner images, which are then transferred onto a recording medium, followed by fixing.
In order to form a clear full-color image through such image formation by full-color electrophotography, it is necessary to keep the amount of the toner on the latent electrostatic image bearing member depending on the latent electrostatic image. The fluctuation of the amount of the toner on the latent electrostatic image bearing member results in fluctuation in image density and color tone on a recording medium. The fluctuation of the amount of the toner on the latent electrostatic image bearing member is due to, for example, fluctuation in charge amount of toner. In particular, in the hybrid developing method, there have been reported that the difference in amount of a toner from place to place on a toner bearing member causes a difference in image density on the next development, which is so-called hysteresis (ghost phenomenon) (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-25693).
In one effective method for overcoming hysteresis in the hybrid developing method, the toner remaining on the toner bearing member after development of the latent electrostatic image on the latent electrostatic image bearing member is removed and then an unused toner is supplied to the surface of the toner bearing member, to thereby compensate the above-described difference in amount of the toner on the toner bearing member. For example, there has been proposed a method of overcoming hysteresis by scraping off the remaining toner from the toner bearing member with a scraper or a toner recovering roller after development and before supply of an unused toner (see, for example, Japanese Patent (JP-B) No. 3356948 and JP-A Nos. 2005-157002 and 11-231652). Also, there has been proposed a method of overcoming hysteresis by recovering the remaining toner from the toner bearing member with a magnetic roller utilizing the difference in electrical potential, to thereby stabilize the amount of the toner on the toner bearing member (see, for example, JP-A No. 07-072733). Furthermore, there has been proposed a method of overcoming hysteresis by providing a wide half-width region between a developer bearing member and a toner bearing member in which region the half value of the peak of the magnetic flux density of a magnetic roller is observed, to thereby stabilize recovery and supply of the toner with respect to the developer bearing member (see, for example, JP-A No. 07-128983). Moreover, there has been proposed a method of overcoming hysteresis by keeping constant the amount of the toner on the toner bearing member where non-spherical carrier particles are used as a carrier of a two-component developer. Specifically, charges are injected into the carrier particles at the tips of the magnetic brushes; the distance between the developer bearing member and the toner bearing member is made small; the amount of the toner to be supplied at one time to the toner bearing member is increased; and the toner is supplied onto the toner bearing member in such an amount that the amount of the toner thereon is saturated (see, for example, JP-A No. 07-092813).
The above-described hysteresis is said to be a specific problem to the hybrid developing method. However, when a developer is used for a long period of time in the two-component developing method, there has been reported that the developing capability of the developer degrades to cause hysteresis where the image density decreases (see, for example, JP-A No. 11-065247).
Hysteresis occurring in the two-component developing method results from inappropriate removal of a two-component developer. The removal of the developer is performed at a removal region having a magnetic force of almost 0, which is formed by an odd number of magnets provided in the developing sleeve and magnets of the same polarity provided below the rotation shaft of the developing sleeve. In the removal region, the developer after development is allowed to freely fall due to gravitational force. However, counter charges are generated on the carrier when the toner is consumed for forming the previous image to thereby form mirror image force between the carrier and the developer bearing member, resulting that the developer cannot normally be removed. Thus, the developer in which the concentration of the toner has been lowered as a result of consumption of the toner is transferred again to the developing region to reduce the developing capability. In other words, the density of the image formed at the first rotation of the sleeve is normal, while the density of the images formed at the second and subsequent rotations is low.
Furthermore, one proposed method of overcoming hysteresis in the two-component developing method is a method of removing the developer after development using magnetic force generated from pumping rollers each having a magnet therein which are arranged near the removal region on the developing sleeve (see, for example, JP-A No. 11-065247). The developer that has been removed is pumped up with another pumping roller and then transferred to a developer-stirring chamber having a screw, where the concentration of the toner is adjusted again and the toner is charged.
However, even with the above-described proposal, the developer is affected by hysteresis after continuously used for a long period of time. Thus, problematically, it is not possible to stably supply a necessary amount of toner to a latent electrostatic image bearing member. In addition, the above proposal cannot reduce an increase in resistance of a carrier caused by toner spent which is a specific problem in the two-component developing method. Therefore, keen demand has arisen for a measure to solve such problems at the same time.